The principles lying behind luminance of light emitting diodes relate to injecting an electric current sequentially through P-N junctions of a semiconductor to generate light, wherein AlGaInP is implemented in high brightness red, orange, yellow and yellowish green LEDs, AlGaInN is in blue and green LEDs. The process of metal organic vapor phase epitaxy (MOVPE) is commonly adopted in the mass production of the LEDs, while the light-emitting components are of the structures, including: homo-junction (HOMO), single-heterostructure (SH), double-heterostructure (DH), single-quantum well (SQW) and multiple-quantum well (MQW) or other appropriate structures.
The structure of a conventional light emitting diode is illustrated in FIG. 1A, including, from the top down, a front electrical electrode, a transparent oxide layer or a window layer 14 to disperse the current, an active layer 12, a substrate 10 and a back contact 13. Among them, the active layer 12 is formed by a light-emitting material, such as AlGaInP or AlGaInN by adopting MOVPE and the transparent conductive oxide layer is a transparent conductive oxide layer typically formed of an indium tin oxide (ITO) layer. After a current is injected through the front contact 11, the current will pass through the transparent conductive oxide layer 14 to disperse and then through the active layer 12 and the substrate 10 to flow towards the back contact 13. Light is emitted when the current flows through the active layer 12. The active layer 12 is a sandwich layer, including a p-type upper cladding layer, an intrinsic layer and an n-type lower cladding layer. However, the low carrier mobility and high resistance of the active layer made of AlGaInP or AlGaInN results in poor electric conductivity of the AlGaInP or AlGaInN. Apart from that, the transparent conductive oxide layer 14 though can improve the current dispersion, its conductive ability is however weaker than a metal layer. Consequently, a metal grid layer 16 is generally embedded in the ITO layer 14, e.g., a metal grid layer formed on the active layer, and then covered it by forming an ITO layer thereover. The current distribution is getting improvement, never less, the primary emitting regions are mainly concentrated at and next to the lower portion of the electrode, as shown in FIG. 1A.
To enhance the current distribution, improvements have been made to the structures and materials, such as that disclosed in U.S. Pat. No. 5,008,718 by Fletcher et al., where a capping layer 15 (or window layer), made of GaP, GaAsP and AlGaAs having a low resistance value and being pervious to light, is added between the front contact and active layer, as shown in FIG. 1B. The objective of using this capping layer is to enhance the current distribution flowing from the front contact. As described in the '718 patent, to improve the current distribution, the capping layer is preferred to be in the range from 150 to 200 micrometers thick to enhance the luminous intensity by 5 to 10 times. However, the increasing thickness of the capping layer also increases the time and cost required for MOVPE epitaxy thereby significantly increasing the cost of the epitaxy. In addition, the distribution ability is extremely relevant to the thickness. Hence, to ensure even current distribution, the thickness must be at least 10 micrometers or the current crowding problem cannot be effectively resolved.
Another embodiment is to change the design of the electrode. The structure metal electrode is a mesh. Please refer to FIG. 1C; a substrate 100 thereover is formed with an active layer, which may be a double hetero junction or a quantum well to improve the emitting efficacy. A transparent layer such as GaP, AlgaAs or an ITO layer 140 is then formed on the active layer to improve the current distribution. The back electrode 130 is formed on the opposite face of the substrate 100. On the other hand, an upper electrode 210, which is a metal mesh formed on the transparent layer 140 and an extra metal pad 110 is formed on the metal mesh 210.
The material of the substrate 100 is dependent on the material of the active layer 120. When the active layer 120 is made of AlGaInP, GaAs is chosen as the substrate. When the active layer 120 is made of AlGaInN. Any of sapphire, may be selected as the substrate. The active layer is preferred to be in the range from 0.3 to 3 μm in thick. The thickness of the transparent layer 140 is preferred to be in the range from 10 to 50 μm. Both the active layer 120 and the transparent layer 140 are formed by MOVPE or molecular beam epitaxy (MBE).
The metallic mesh layer 210 suggested are dimensioned to 0.5 to 5 micrometers and evenly distributed above the substrate. If the meshes are dimensioned to 2 μm with a capping layer having a thickness of 15 μm, the light-emitting angle θ is calculated by, tan s2θc=2/15→2θc≈7.6°→θc≈3.8°.